1. Field of the Invention
The present invention relates to an energy trap-type piezoelectric resonator. More specifically, this invention relates to a piezoelectric resonator having an energy trap-type piezoelectric vibrating section and a capacitor including a pair of mutually opposed capacitor electrodes. Further, the invention also relates to a chip-type piezoelectric resonant element formed by using such piezoelectric resonators.
2. Description of the Related Art
Conventionally, in an intermediate frequency band of a mobile communication device such as an FM receiver and a portable telephone, a piezoelectric filter has generally been used as a frequency band filter.
Japanese Unexamined Patent Application Publication No. 10-284985 discloses one example of the above described piezoelectric filter. In the following, such a kind of a conventional piezoelectric filter will be described in detail with reference to FIG. 7 and FIG. 8.
As shown in FIG. 7, the conventional piezoelectric filter is constructed by using a first piezoelectric substrate plate 71 and a second piezoelectric substrate plate 72. On the upper surface of the first piezoelectric substrate plate 71, resonant electrodes 73a and 73b are provided. Further, on the underside surface of the first piezoelectric substrate plate 71, another resonant electrode serving as a common electrode is provided in a position opposite the above electrodes 73a and 73b. By virtue of the resonant electrodes provided on these main surfaces of the substrate plates, an energy trap-type piezoelectric vibrating section is provided which functions in a thickness-extension oscillation mode.
Further, a capacitor electrode 75a is connected with the resonant electrode 73b. Particularly, the capacitor electrode 75a is provided along an edge 71a of the piezoelectric substrate plate 71.
Moreover, a resonant electrode 73c serving as a common electrode is provided on the upper surface of the second piezoelectric substrate plate 72. In addition, the resonant electrode 73c is electrically connected with a capacitor electrode 75b. Similarly, the capacitor electrode 75b is provided along an edge 72a of the piezoelectric substrate plate 72. Here, the piezoelectric substrate plate 71 and the piezoelectric substrate plate 72 have the same structure. Namely, on the bottom surface of the piezoelectric substrate plate 71 an electrode structure is provided which is the same as the electrode structure provided on the upper surface of the piezoelectric substrate plate 72. In contrast, on the bottom surface of the piezoelectric substrate 72 an electrode structure is provided which is the same at the electrode structure provided on the upper surface of the piezoelectric substrate plate 71. Therefore, with respect to the same electrodes, the same reference numerals are used to represent similar elements on the two piezoelectric substrate plates 71 and 72.
With such a piezoelectric filter, the first piezoelectric substrate plate 71 and the second piezoelectric substrate plate 72 are laminated one above the other such that the resonant electrodes 73c serving as the common electrodes face each other. Here, spacers 77 to 79 are provided having circular holes 77a to 79a, respectively, provided such that the oscillation of the piezoelectric oscillating section is not suppressed. Further, sealing substrate plates 80 and 81 are provided.
Moreover, in the above described piezoelectric filter a circuit configuration shown in FIG. 8 is employed. Namely, the first and second piezoelectric substrate plates 71 and 72 of the piezoelectric filter section are electrically connected by relay capacitors. Such relay capacitors are provided on the piezoelectric substrate plates 71 and 72 by an electrostatic capacitance existing between the capacitor electrodes 75a and 75b. 
Furthermore, in the above described piezoelectric filter, the circuit configuration shown in FIG. 8 has been employed, but the piezoelectric filter section has been divided into first and second piezoelectric filter sections in the first piezoelectric substrate plate 71 and the second piezoelectric substrate plate 72, respectively. Since the first piezoelectric substrate plate 71 and the second piezoelectric substrate plate 72 are laminated one above the other, the actual mounting area is reduced.
However, to obtain the first piezoelectric substrate plate 71 and the second piezoelectric substrate plate 72 described above, a piezoelectric substrate plate 82 shown in FIG. 9 must be prepared in the conventional art. By dividing the piezoelectric substrate plate 82, the first piezoelectric substrate plate 71 and the second piezoelectric substrate plate 72 are obtained.
However, during an actual manufacturing process, in order to ensure improved productivity, a mother piezoelectric substrate plate including several pieces of the piezoelectric substrate plates 82 arranged in parallel with the main surface of each substrate plate has been used.
By dividing the piezoelectric substrate plate 82 along dotted chain lines Bxe2x80x94B in FIG. 9, the first and the second piezoelectric substrate plates 71 and 72 are obtained. At this time, the capacitor electrodes 75a and 75b of the piezoelectric substrate plates 71 and 72 are thus formed through such a cutting process.
On the other hand, when the above cutting position deviates from its correct position, the capacitor electrode 75a provided on one piezoelectric substrate plate 71 will be different in size from the capacitor electrode 75b provided on the other piezoelectric substrate plate 72 in a direction orthogonal to the cutting line.
Therefore, in order to divide the piezoelectric substrate plate 82 into two portions, it is necessary that the piezoelectric substrate plate 82 be very precisely divided.
However, in practice, the cutting of the piezoelectric substrate plate 82 into two portions with high precision is difficult, and often impossible to avoid differences in the size between the two capacitor electrodes 75a and 75b provided on the piezoelectric substrate plates 71 and 72.
For the above reason, if a piezoelectric filter is produced having the above described structure, the filter characteristics of one piezoelectric filter will be different from those of another.
In order to overcome the problems described above, preferred embodiments of the present invention provide an improved piezoelectric resonator including an energy trap-type piezoelectric vibrating section and a capacitor section provided by two capacitor electrodes arranged to face each other with a piezoelectric plate interposed therebetween. In particular, preferred embodiments of the present invention provide an improved piezoelectric resonator which ensures that the capacitance of a capacitor section does not differ from one piezoelectric resonator to another, so as to produce desired resonance characteristics and desired filter characteristics with high precision. Further, preferred embodiments of the present invention provide a piezoelectric filter.
According to preferred embodiments of the present invention, an energy trap-type piezoelectric resonator is provided which includes a piezoelectric substrate plate having a first main surface and a second main surface, first and second resonant electrodes provided on the first and second main surfaces of the piezoelectric substrate plate, respectively, to form an energy trap-type piezoelectric vibrating section, and first and second capacitor electrodes provided on the first and second main surfaces of the piezoelectric substrate plate, respectively, and arranged one above the other with the piezoelectric substrate plate interposed therebetween. In particular, the first and the second capacitor electrodes extend along the first and the second main surfaces to the edges thereof. Further, the first capacitor electrode is spaced by a desired margin area from an adjacent outside edge of the piezoelectric substrate plate, except at lead portions led out to said adjacent outside edge.
Preferably, the first capacitor electrode is spaced from the adjacent outside edge of the piezoelectric substrate plate such that the distances between the outer edge of the first capacitor electrode itself and an outer edge of the second capacitor electrode, in a direction in which the first capacitor electrode is spaced, are at least equal to the thickness of the piezoelectric substrate plate.
Preferably, the piezoelectric resonator is an energy trap-type piezoelectric filter, the first resonant electrode includes a pair of resonant electrode members provided on the first main surface of the piezoelectric substrate plate, with a desired gap provided between the pair of electrode members, the second resonant electrode is a common electrode which is provided on the second main surface of the piezoelectric substrate plate such that said second resonant electrode is located opposite the first resonant electrode provided on the first main surface of the substrate plate, and the first and the second capacitor electrodes are electrically connected to the first or the second resonant electrodes, respectively.
It is preferable that the two piezoelectric resonators forming the above piezoelectric filter are laminated one above the other such that vibrations of both piezoelectric vibrating sections of the two resonators are not suppressed and that the common electrodes face each other.
Preferably, the region of said piezoelectric substrate plate where the first and the second capacitor electrodes face each other is non-polarized.
According to another preferred embodiment of the invention, a chip-like piezoelectric resonant element is provided which includes a piezoelectric resonator as described above, and first and second case substrate plates each having a recess portion which is provided such that the vibration of the piezoelectric resonator is not suppressed, said first and second case substrate plates being arranged one above the other with the piezoelectric resonator interposed therebetween.